An image sensor, which is a semiconductor device that converts an optical image into an electrical signal, is largely classified as a charge coupled device (CCD) image sensor or a Complementary Metal Oxide Semiconductor (CMOS) image sensor.
The CMOS image sensor sequentially detects electrical signals of respective unit pixels in a switching scheme by utilizing photodiodes and MOS transistors in unit pixels, thereby displaying an image.
A CMOS image sensor can include a photodiode region for receiving a light signal to convert it into an electrical signal and a transistor region for processing the electrical signal.
The typical CMOS image sensor has a structure where the photodiode and the transistor are horizontally arranged.
Many disadvantages of the CCD image sensor are addressed by the horizontal type image sensor; however, there are still problems associated with the horizontal type image sensor.
In particular, the photodiode and the transistor of a horizontal type image sensor are formed to be horizontally adjacent to each other on a substrate. Accordingly, an additional region of the substrate for forming the photodiode is required for each unit pixel so that the region of a fill factor is reduced and the resolution may be limited.
In addition, according to the horizontal type image sensor, it can be very difficult to achieve the optimization for a process simultaneously manufacturing the photodiode and the transistor. For example, a shallow junction is required for a low sheet resistance in a rapid transistor process; however, such a shallow junction may not be appropriate for the photodiode.
To add additional on-chip functions to the horizontal type CMOS image sensor, the size of a unit pixel may need to be increased or decreased, depending on circumstances, in order to maintain sensitivity of the horizontal type CMOS image sensor.
However, when the size of the unit pixel is increased, the resolution of the horizontal type CMOS image sensor is decreased. In addition, when the area of the photodiode is decreased, the sensitivity of the horizontal type CMOS image sensor is decreased.